N. Frini- Srasra1,2,*E.Srasra1
DETERMINATION OF ACID–BASE PROPERTIES OF HCL ACID
ACTIVATED PALYGORSKITE BY POTENTIOMETRIE TITRATION
1
Unité de recherche sur les matériaux,CRTE,
Technopole Borj Cedria ,B.P.95-2050 Hammam-Lif Tunisia, [email protected]
2
Département de Chimie, Faculté de Sciences de Tunis, 1060 Tunis, Tunisia
Introduction
Clay minerals have many applications related to adsorption and catalysis, which use the acidic-basic
properties of the clay mineral surface. Their textural and structural properties may lead to a wide range of
applications especially if activation is able to increase the number and strength of its surface acidic sites.
There are some studies on the acid activation of palygorskite: [1] investigated the effect of magnesium on
structural alteration and on specific surface area and porosity, and proposed ([2]) a mechanism of acid activation of magnesic palygorskite. [3] studied the structural and textural modification of palygorskite under acid
treatment in comparison with a Spanish sepiolite treated under the same experimental conditions. [4] studied
the influence of octahedral sheet composition on the kinetics of acid leaching of palygorskite [5, 6], showed
that Tb3+and La3+ increase acid sites number and improve catalytic properties of acid activated palygorskite.
Activated palygorskite presents an excellent catalytic activity in the rearrangement of chalcone epoxide reaction ([7]). The rearrangement of α-epoxyketone (chalcone epoxide) was found to yield E(-) enolic form of
1,3-diphenyl-1,3-propanedione indicating that the proton migration has taken place from the less favoured
direction of the epoxy ring. It was observed, that acid activated palygorskite give higher yield (90%) of the
compound; while, raw palygorskite, kaolinite as well as zeolite 4A have no effect in the rearrangement of
chalcone epoxyde.
Although, the properties of acid activated palygorskite has been studied from different point of view,
no attempt has been made to study the charge characteristics and especially the point of zero charge (PZC)
and the acidic surface constant (pKa). The determination of PZC based on potentiometric titrations or by
mass titration has been the subject of many studies on oxide minerals as well as on layer clay minerals
[8−13]. However similar studies for fibrous clay are missing. The PZC of palygorskite and sepiolite was
evaluated, until now, only by electrophoretic mobility measurements in dilute suspensions in order to determine the isoelectric point [14, 15].
In this study, we determined the acid base properties on a series of acid activated palygorskite samples. The total surface site and the surface site density were calculated from Gran plots [16]. The acidic surface constants were estimated with three surface protonation models at a specific ionic strength. The point of
zero charge was evaluated using the common crossing point of Z vs pH curves performed at different ionic
strengths. The effect of activation period on acid-basic properties of palygorskite was investigated.
Material and Methods
Red palygorskite from south Tunisia was used as starting material. The crude sample contains impurities mainly dolomite (8%, measured by calcimetry) and less than 5% of quartz. Crude palygorskite was
firstly treated with diluted HCl solution in order to remove carbonate. Then, it was washed with distilled water by centrifugation and dialysis to remove excess chloride. After sedimentation, the purified fraction was
dried and grinding. The average size of the grains is <60 µm. Referring to ASTM file (Reference code: 210957), Tunisian palygorskite has monoclinic structure (Figure 2). Chemical composition of purified sample
is (SiO2 53.5%, Al2O3 11.98%, MgO 12.05%, Fe2O3 5.9%, K2O 0.72%, Na2O 0.03%, CaO 0.09%, LOI 16%
of lost weight) and its structural formula, calculated according to [17], is as follows:
(Si7.324Al0.676)(Mg2.459Al1.257Fe0.608)(K0.125Na0.008Ca0.013)O20(OH)2(H2O)4 4(H2O). The cation exchange capacity
(CEC) carried out by Kjeldahl method is 23 meq/100g and its specific surface area is 59.7 m2g-1.
_______________________________________________________________________________________
© Frini- Srasra N., Srasra E., Электронная обработка материалов, 2008, № 5, С. 69−78.
69
The purified palygorskite sample was subjected to acid treatment with 2M hydrochloric acid using
solid/liquid ratio of 10/100 (w/w) at boiling temperature under reflux for different periods. The resulting solids were washed with distilled water until chloride free and pH was constant.
That Activated and raw palygorskite were characterised by X ray diffraction (Pan analytical X’Pert
High Score Plus diffractometer equipped with a Cu anticathode), chemical analysis [18] and specific surface
area (Micrometric Autosorb 2050 Analyser), from the nitrogen adsorption isotherms at 77 K by the BET
method and after degassing the samples at 150°C with the residual pressure of 10-5 mm Hg.
Acid-base properties of palygorskite suspensions were determined using Potentiometric titration.
Measurements were performed with a micro burette containing the titrant (NaOH 5.13⋅10-2M) and a HI 9321
microprocessor pH meter (HANNA Instruments) combination electrode, calibrated with two commercial pH
buffers at ambient temperature and in an aerated medium.
For all acid-base titrations, 0,1 g palygorskite was added to a 15 ml water flask and stirred for 24
hours in order to attain equilibrium. Palygorskite suspensions were firstly acidified by HCl 10-2M at pH ca 2
and then titrated with hydroxide solution to an alkaline pH (pH =11). NaCl solution was used to stabilize the
system at a fixed ionic strength. Distilled water was added to bring the total initial volume of the suspension
to 50 ml. The temperature was held constant at 25 ± 0.5°C. Afterwards, 5.13.10-2 M NaOH in 0.2 ml increments was used to titrate the suspension up to a pH approximately 11. The equilibrium criterion for each addition of the titrant was the stability of pH value measured. The NaCl electrolyte concentration was adjusted
to 0.1, 0.01 and 0.001 mol⋅L-1. Before each potentiometric titration the aqueous suspensions were equilibrated for about 10 min to each equilibrium pH value.
Theorhical al Bakground
For each studied system, Gran plots for the hydroxide titration are made from the experimental data
and the two equivalence points Ve1 and Ve2 values was determined by linear regression as shown in
Figure 1. The Ve1 values obtained in the acidic region are used to calculate the amount of total protons added
to the system. In fact, before the Ve1 point the added NaOH reacted with the excess of HCl put initially in the
system and so we can consider the Ve1 point as the zero titration point of the palygorskite surface. Therefore,
for each titration point, the concentration of total protons added is determined by:
H TOT =
−(Vb − Ve1 )Cb
(V0 + Vb )
(molL-1)
(1)
Where Vb is the volume of NaOH added, V0 is the initial volume of the system and Cb is the concentration of
NaOH.
Figure 1. Gran plot of Paly2M25h suspension in 0.1M NaCl
The total surface site per solid weight (Hs) calculated from the two equivalence points in the Gran
plot Ve1 and Ve2 and the average number of protons reacted per surface site (Z) were defined by the following formula:
70
Hs =
Z=
(Ve2 -Ve1 )Cb
ms
(mol g-1)
[2]
( H TOT − 10 − pH + 10 ( pH − pK w ) ) × (V0 + Vb )
H s × ms
[3]
Where ms is the solid weight used in the titration system.
The surface acidic constants were determined graphically. Three surface protonation models were
considered in this study. The one site-one pKa model (model I), the two sites-two pkas model (model II) and
the one site-two pKa model (model III).
Models I and II
For the two models I and II, we assume that the amphoteric surface hydroxyl groups (≡SOH) only
release protons to form negatively charged surface sites (≡SO-) as shown in the following equation.
≡SOH
'
≡SO+ H+
Ka
Ka =
[≡ SO ] [H ]
−
+
pH = pK a + log
and from this
[≡ SOH ]
[≡ SO ]
−
[≡ SOH ]
If we consider the dissociation coefficient of (≡SOH) (α) and the electrostatic effects the pH formula becomes:
pH = pK a + n log
α
1− α
[4]
Where n is the Henderson-Hasselbach empiric constant and α is expressed by: α =
[≡ SO ] . To
[≡ SOH ] + [≡ SO ]
−
−
determine the ≡SO- concentration we considered the charge balance equation and the Gran plot data.
[H+] + [Na+] = [Cl-] + [OH-] + [≡SO-]
[Na + ] =
C b Vb
(V0 + Vb )
[
]
Hence, ≡ SO − =
and
(Vb − Ve1 )Cb
(V0 + Vb )
[Cl − ] =
C b Ve1
(V0 + Vb )
+ [ H + ] - [OH - ]
[5]
By substituting Eq [1] into Eq [6], [≡SO-] can be expressed by:
[≡SO-] = – HTOT + 10-pH – 10(pH-pKw)
By using the Z formula (Eq [3]) the ≡SO- concentration can be rewritten as follows:
[≡ SO ] = − Z(×V H+ V× m)
−
s
0
s
[6]
[7]
b
The determination of the total surface site ([≡SO-] + [≡SOH]) differs from the surface protonation model.
¾ For the one site-one pKa model, we assume that amphoteric surface hydroxyl groups (≡SOH) are
homogeneous;
[≡ SOH ] + [≡ SO − ] =
H s × ms
(V0 + Vb )
[8]
⇒
α=
[≡ SO ]× (V
−
0
+ Vb )
H s × ms
[9]
and by substituting Eq [7] into Eq [9] we obtain
α = -Z
[10]
¾ For the two sites-two pKas model, we assume the existence of two types of sites a) weak acidic
sites with a concentration of sites [Wa] which dissociate at pH<7, and b) weak basic sites with [Wb] site concentration which dissociate at pH >7.
≡SIOH
'
≡SIO+ H+
Ka1
≡SIIOH
'
≡SIIO
+ H+
Ka2
Moreover, when the interface between clay mineral and water has different acidic sites, the successive dissociation of these sites can be defined by their dissociation coefficients α1 and α2 corresponding to pKa1 and
pKa2 respectively:
α1
[≡ SO ]
=
−
[Wa ]
⎡⎣ ≡ SO − ⎤⎦ − [Wa ]
α2 =
[Wb ] + [Wa ]
where [Wa] is the concentration of weakly acidic sites between pH 5-7, determined by:
71
[Wa ] =
(Ve − Ve 1 )C b
[11]
(V0 + Vb )
[Wb] is the concentration of weakly basic sites at pH > 7, determined by:
[Wb ] =
(Ve2 − Ve )C b
(V0 + Vb )
[12]
Ve is the equivalence point determined by the maximum of the differential curve dpH / dV. From Eqs [7]
[11] and [12] α1 and α2 can be calculated.
α
α
Finally, by extrapolating the linear regression curve pH vs. log
to zero log
, we can esti1−α
1−α
mate the pKa value.
Model III
For model III or the One site amphoteric surfaces model, reasoning and assumptions will be different. An amphoteric site is bifunctional, it can serve both as an acidic and as a basic functional group. It can
undergo protonation and deprotonation
≡SOH + H+ ' ≡SOH2+
Ka1
+
≡SOH ' ≡SO + H
Ka2
The corresponding equilibrium constants are
K a1 =
[≡ SOH ] [H + ]
K a2 =
[≡ SOH ]
+
2
[≡ SO ] [H ]
−
+
[≡ SOH ]
A mass balance on the surface sites can be written as:
[≡SOH ]TOT = [≡SOH] + [≡SO- ] + [≡SOH2+ ]
[13]
+
+
+
The charge balance equation is : [H ] + [Na ] + [≡SOH2 ] = [Cl ] + [OH ] + [≡SO ] [14]
The surface charge Q is defined by the difference between the positively and negatively charged
sites
Q = [≡SOH2+] - [≡SO-] = [Cl-] - [Na+] + [OH-] - [H+]
Q = HTOT + [OH-] - [H+]
Z × Hs × ms
(V0 + Vb )
Q=
[15]
At pH<< pHPZC, we can neglect the deprotonation (the solid surface is positively charged) and equation [13]
becomes [≡SOH ]TOT = [≡SOH] + [≡SOH2+] and Q = [≡SOH2+].
On the other hand, When [H+] is very low (at pH>> pHPZC) the surface acquires a net negative
charge and equation [13] becomes [≡SOH ]TOT = [≡SOH] + [≡SO- ]
and
Q = - [≡SO- ]
Under these conditions, the Ka1 and Ka2 approximate to:
K a1 =
([≡ SOH ]TOT
[ ]
- Q) H +
K a2 =
Q
[ ]
−Q H+
[≡ SOH ]TOT + Q
By using Eq. [8] and [15] we obtain:
K a1 =
(1 − Z ) [H + ]
K a2 =
and
Z
[ ]
−Z H+
1+ Z
And from these
⎛1− Z ⎞
⎛1+ Z ⎞
pH = pK a1 + n log⎜
⎟ pH = pK a2 − n log⎜
⎟
⎝ Z ⎠
⎝ −Z ⎠
⎛1− Z ⎞
⎛1+ Z ⎞
By extrapolating the linear regression curves of pH versus log⎜
⎟ and versus log⎜
⎟ , we can esti⎝ Z ⎠
⎝ −Z ⎠
mated the pKa1 and pKa2 values.
K K
a1
a2
=
[≡ SO ] [H ]
[≡ SOH ]
+ 2
−
+
2
pH =
pK a1 + pK a2
2
-
+
[
]
n
≡ SO −
log
2
≡ SOH 2+
[
]
+
Near pHPZC, [≡SOH] is negligible in comparison with [≡SO ] + [≡SOH2 ].
From this approximation, we obtain
pH =
pK a1 + pK a2
2
+
1− Z
n
log
2
1+ Z
[16]
Results and Discussion
Acid activation
The X ray diffractograms of purified and acid treated samples are presented in Figure 2. The untreated palygorskite has sharp strong reflexions at 10.5, 6.4, 5.4, 4.5, 3.68 and 3.23Å. With increasing treating time, the crystallinity of palygorskite decreased, with a concomitant formation of amorphous utter. The
latter can be observed by the appearance and increase of a broad hams at 16-30° 2θ. After 35h treatment only
this hams is observed indicating the complete destruction of the clay.
Figure 2. XRD diffractograms of treated and untreated palygorskite
Acid treatment caused a progressive decrease in octahedral cations (Al, Mg and Fe) and a residual
enrichment in SiO2 (Table 1). In the beginning of acid treatment, the decrease in octahedral cations is limited
showing the resistance of palygorskite structure and the difficulties of the opening of the channels. Once the
H+ access becomes easy throughout the structure is facilitated, the dissolution becomes rapid and reaches its
maximum at 35h. For greater activation times, we note an increase in the octahedral cations, which can be
attributed to coprecipitation.
Table 1. Chemical compositions of untreated and acid treated samples expressed in oxide form/100g of calcined sample
Samples
SiO2 Fe2O3 MgO Al2O3
Paly
63.69 7.023 14.34 14.26
Paly2M1h 69.72 6.11
12.54 11.96
Paly2M8h 71.4 5.96
11.89 11.09
Paly2M25h 75.37 4.27
11.18 9.55
Paly2M35h 88.08 0.66
4.60 7.08
Paly2M40h 80.90 3.37
8.12 8.01
Paly2M50h 79.58 3.79
5.76 8.57
BET specific surface area gradually increases with treating time up to 35h (from 59.7 m2/g to 437
m /g) decreasing thereafter (Table 2). Such behaviour has been observed in the past [19, 20, 3]. The initial
increase of SBET is attributed to the opening of the structure channels following dissolution of the octahedral
sheet and amorphous silica formation; whereas the decrease can be attributed to the condensation of the silanol groups [2]. This confirms that silica contributes to the total amount of the surface area. Moreover, activation increases the total pore volume from 0.3494 to 1.039 cm3g-1 due to development of mesoporosity.
Acid-base properties of acid treated and untreated palygorskite
Determination of total surface site
Hs average values are listed in Table 2. The total surface site of Tunisian palygorskite determined by
potentiometric method is 0.518 mmol g-1, comparable to a Brazilian palygorskite (0.476 mmol g-1) determined by N-butylamine thermodesorption [21].
As expected, Hs increases with time of acid activation to 1.53 mmolg-1 at 35h, decreasing slightly
thereafter.
The site density values (Ds) show that there is no correlation between the total surface site (Hs) and
the surface site area SBET (Table 2).
Surface charge
To determine the surface charge of our samples versus pH, we considered the average number of
protons reacted per surface site (Z) determined by equation [3].
Figure 3 shows the evolution of Z versus pH for the different samples. Acid activation modifies the
shape of the curves as well as the sign of surface charge. Moreover, the behaviour of Z vs pH curves is not
the same for all treated samples. Potentiometric titration shows a negatively charged surface in all pH range
(3-11) for the untreated palygorskite and for the samples treated for longer time (Paly2M35h Paly2M40h and
Paly2M50h). Such behaviour is similar to that observed for illite [22] and for amorphous SiO2 [23]. This
means that within the entire pH range, deprotonation reaction is predominant. Furthermore, in the case of
natural palygorskite the Z plots reach a plateau at Z (pH, I) approximately - 0.65, which indicated that more
than half of the total surface sites have released their protons. This suggests that the surface is heterogeneous
and there are mainly two kinds of surface sites with different hydroxide affinities at the solid/water interface.
This plateau is not pronounced for the treated palygorskite samples. When the palygorskite is moderately
treated, its surface shows a different behaviour. Depending on pH, its surface can bear net negative or positive or no charge. Therefore, the ≡SOH groups at external surfaces can be protonated and deprotonated under
acidic and alkaline pH conditions respectively. The acid activation produces new surface sites having different acid-basic properties.
Determination of zero charge (PZC)
Curves of Z versus pH at different NaCl concentration plotted for different samples. The curves have
a common intersection point, at a pH which is considered as a point of zero charge (PZC). As shown in
Figure 3 and Table 2, the PZC decreases gradually from about 8.8 in the starting material to about 3.9 in the
most heavily treated sample Paly 2M50h. The amount of this shift depended on the acid treatment period.
We can conclude that acid treatment produces a shift of the PZC of the sample toward the PZC of the amorphous silica (at about 2-3) [24]. This result is consistent with the fact that Paly 2M50h sample contained 80%
of amourphous silica and that no residual palygorskite is present.
Determination of the surface acidic constants
To explain the behaviour of the surface hydroxyl groups three surface protonation models was used.
The choice of models is based on the surface charge results. Model I and II are applied when Z is always
negative at the entire pH range. In the both models, we considered that only deprotonation reaction is occurring at the solid surface. Model III is used when surface sites can undergo protonation and deprotonation.
Thus, we tested model I and II for natural palygorskite. Model I was tested for Paly2M35h, Paly2M40h and
Paly2M50h and model III for Paly2M1h, Paly2M8h and Paly2M25h.
We should note that for all the previous calculation, a weakly acidic surface functional group (≡XH)
accounting for ion exchange reactions, is neglected because palygorskite has a low cation exchange capacity.
Moreover, total number of structural-charge sites which are accessible for Na/H exchange reactions do not
exceed 2% of the total CEC [25].
pKa values estimated with different surface protonation models are summarised in Table 3 and fits
of the experimental data to the models are shown in Figure 3. For the starting material we obtained a pKa
value between 6.47-6.85 with the one site-one pKa model and pKa1=5.26-5.9, pKa2 = 10.05 -10.64 respectively for the two surface acidic constants when we use the two sites-two pKa model. If we compare the pKa
values obtained from the two models, we see clearly that the pKa in model I is comparable to the pKa1 in
model II. Model I simplifies the heterogeneous palygorskite surface as a system with uniform acid-base
2
properties and simulates the surface acid-base characteristics with the behaviour of the stronger surface sites.
However, model II gives better description of the experimental data than model I. Unlike the natural palygorskite, the model I is suitable for the samples treated for a long time (more than 25 hours). pKa values obtained from this model is 8.7−9.3. Application of model III in acid treated samples for a period less than 35
hours, yielded two pKa values: pKa1 in the range 3−4 and pKa2 at about 9.5. If we compare these results with
the pKa values of silanol and aluminol groups found in the literature (Table 4), we can conclude that in the
natural palygorskite there are two kinds of surface sites aluminol (pKa ≈ 5) and silanol sites (pKa ≈ 10).
Acid treatment progress disrupts the 2:1 structure and produces incomplete tetrahedra and octahedra at the
edges. Opening of channels allows the H+ access to acidic basal AlOH sites (pKa ≈ 3.4) stronger than edges
AlOH sites (pKa ≈ 6.5). Beyond 25 hours of acid treatment, acid activation palygorskite behaves as amorphous silica (pKa ≈ 9).
Figure 3. Potentiometric titration curves for different palygorskite samples for each ionic strength.Fits of the
experimental data to the models are shown for suspension samples in 0.001M NaCl
Table 2. Surface area, Total surface site surface, site density and point of
skite samples
Time (h)
0
1
8
25
35
Hs (mmol g-1) 0.518 0.758 1.003 1.029 1.527
SBET (m2g-1)
59.7 127.5 161.6 307.2 437.0
Ds *(sites nm-2) 5.22 3.58 3.74 2.02
2.1
PZC
8.9
7.1
6.8
6.3
5.8
* Ds =
(H s × N A )
S BET × 10
18
zero charge of different palygor40
50
1.435 1.425
325.3 138.3
2.66
6.2
4.5
3.9
(sites nm-2); Where: NA is the Avogadro's number (6,02 .1023 mol-1) and
SBET the palygorskite N2/ BET surface area expressed in m2g-1.
Table 3. pKas values estimated with different surface protonation models
pKa1 - pKa2 - (pKa1 + pKa2)/2
pKa
pKa1 - pKa2
Model I
Model II
Model III
0.1M 0.01M 10-3M 0.1M 0.01M 10-3M 0.1M
0.01M
10-3M
6.68
Paly
6.47
6.85
5.26
5.46
10.13 10.64
Paly2M1h
5.89
10.05
4.9
3.98
3.47
9.28
9.29
9.42
7.05 (6.88) 6.64 (6.05) 6.45 (7.00)
Paly2M8h
3.38
3.32
9.32
9.49
6.35 (6.70)
6.4 (6.59)
Paly2M25h
3.01
3.11
9.51
9.37
6.26 (6.90) 6.24 (5.87)
Paly2M35h 9.04
8.84
Paly2M40h 9.18
Paly2M50h 8.69
pK a1 + pK a2
2
8.84
9.30
8.91
8.69
Values between parentheses are obtained from Eq [16] by extrapolating the linear regression
⎛1− Z ⎞
⎟
⎝1+ Z ⎠
curves of pH versus log⎜
Table 4. Surface dissociation constants for some aluminium and silica of kaolinite compounds found in the
literature
pka1
pka2
References
AlOH
2.9
9.84
Schindler et al., 1987
SiOH
1.0
4.0
Carroll-Webb and Walther, 1988
AlOH
7.9
9.1
Carroll-Webb and Walther, 1988
SiOH
2.4
6.5
Motta and Miranda, 1989
AlOH -Basal
3.4
8.4
Wieland and Stumm, 1992
AlOH- edges
6.5
8.5
Wieland and Stumm, 1992
8.23
Brady and al., 1996
5.28
Brady and al., 1996
SiOH
AlOH-edges
2.33
Conclusion
The potentiometric titration showed that acid treatment produced an increase in the total surface site
number, a shift of the PZC of the sample toward the PZC of amorphous silica depending on the acid treatment period and changes on the surface charge properties. Thus, the modification of palygorskite by acid
activation creates new active sites and increases its catalytic properties (such activity, selectivity and thermal
stability) and adsorption capacity.
Application of the three surface protonation models leads to reasonable descriptions of the surface
properties and their evolution with acid treatment.
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Received 15.01.08
Summary
The surface acidity of raw and acid activated palygorskite clay were studied by acid-base potentiometric titration. The Gran plot method was applied for the hydroxide titration and the total surface site (Hs)
and the average number of protons reacted per surface site (Z) of palygorskite samples at a given ionic
strength were calculated. Acid treatment increases the clay acidity and modifies its surface charge. The point
of zero charge value, determined by the common crossing point of Z vs pH curves performed at different
ionic strengths, decrease from 8.8 to 3.5 with acid treatment period. For illustrating the acidic characteristics
of treated and untreated palygorskite surface, three surface protonation models are tested : a/ the one site-one
pKa model, ≡SOH ' ≡SO- + H+, b/ the two sites-two pKas model, ≡SIOH ' ≡SIO- + H+ and ≡SIIOH '
≡SIIO- + H+ and c/ the one site-two pKa model, ≡SOH ' ≡SO- + H+ and ≡SOH + H+ ' ≡SOH2+. The
three surface protonation models describe sufficiently of the surface properties and their evolution with acid
treatment.
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